1. Technical Field
The present invention relates to a media stripper mechanism, and more particularly, to a mechanism for stripping a recording medium from a rotary member, which may be employed in an electrophotographic image forming apparatus, such as a photocopier, facsimile machine, printer, plotter, or multifunctional machine.
2. Background Art
In electrophotographic image forming apparatuses, such as photocopiers, facsimile machines, printers, plotters, or multifunctional machines incorporating several of these imaging functions, an image is formed by attracting toner particles to a photoconductive surface for subsequent transfer to a recording medium such as a sheet of paper. After transfer, the imaging process may be followed by a fixing process using a fixing device, which permanently fixes the toner image in place on the recording medium by melting and setting the toner with heat and pressure.
Various types of fixing devices are known in the art, most of which employ a pair of generally cylindrical looped belts or rollers, one being heated for fusing toner (“fuser member”) and the other being pressed against the heated one (“pressure member”), which together form a heated area of contact called a fixing nip, through which a recording medium is passed to fix a toner image onto the medium under heat and pressure.
One important factor that determines imaging quality of a fixing device is the ability to convey a recording medium through the fixing nip without causing the recording medium to wind or wrap around the rotary fixing member. Media wraparound occurs where the toner image heated through the fixing nip becomes sticky and thus adheres to the surface of the fixing member upon exiting the fixing nip. If not corrected, a recording medium wrapping around the fixing member would cause jam or other conveyance failure in the fixing nip.
Several approaches have been employed to prevent toner adhesion to the fixing member and concomitant conveyance failures. Some use a special type of toner containing wax additives; others cover the surface of fixing roller or belt with a layer of non-stick, release material or with a coating of silicone oil or other suitable lubricant; and still others employ a media stripper that mechanically strips a recording medium from the fixing member. The media stripper mechanism, for example, may include one or more stripping fingers which defines a stripping edge that contacts the leading edge of the recording medium to strip it from the fixing member, as well as a guide surface along which the recording medium is guided after stripping from the fixing member.
One problem encountered when employing the media stripper mechanism in the fixing device is that forcing the recording medium with the stripping finger causes the guide surface of the stripping finger to interfere with the toner image printed on the recording medium. Not surprisingly, such interference would adversely affect imaging and conveyance performance of the fixing device, where the toner image, which is in a hot, molten state immediately after thermal fixing, is scratched by contacting the guide surface to form linear streaks on the resulting print, or otherwise, sticks to the guide surface to hinder proper conveyance of the recording medium.
The problem is particularly pronounced where printing is performed using coated paper, an increasingly popular type of recording medium typically formed of a paper substrate having a coating of resin on its surfaces. Because of the resin surface coating becoming soft when heated to a process temperature, the coated paper does not provide sufficient stiffness to counteract adhesion forces between the toner image and the fixing member at the exit of the fixing nip, resulting in an increased risk of the toner image to contact or stick to the guide surface.
Various methods have been proposed to provide an effective media stripper mechanism to prevent adverse effects due to interference between the recording medium and the stripping finger.
For example, one such method utilizes a plurality of stripping fingers having different configurations of the guide surface depending on the position at which each stripping finger is disposed in an axial, longitudinal direction of the fixing assembly. In this media stripper mechanism, the stripping finger positioned at a longitudinal end of the fixing member defines a relatively convex guide surface curving outward toward the media conveyance path whereas the stripping finger positioned at a longitudinal center of the fixing member defines a relatively concave guide surface curving inward away from the media conveyance path.
Although generally successful for its intended purpose, the method described above has several drawbacks. One drawback is that varying the configuration of the guide surface in the longitudinal direction results in a localized, concentrated pressure with which the recording medium is pressed against the stripping finger at the longitudinal end of the fixing member. Another drawback is that the guide surface can become irregular or uneven due to buildup of toner deposits resulting from repeated contact between the guide surface and the toner image on the recording medium being guided.
Concentrated pressure on the recording medium and increased irregularities of the guide surface both would adversely affect proper conveyance of the recording medium, or aggravate image defects due to contact between the toner image and the guide surface. These drawbacks make the media stripper mechanism less effective than would be desired, particularly where soft, coated paper is used.